Tool steel for hot working

ABSTRACT

A tool steel for hot working containing, in weight percent, from 0.25 to 0.60% carbon, not more than 0.6% silicon, from 0.50 to 1.50% manganese, from 0.50 to 1.50% nickel, from 1.50 to 3.50% chromium, from 2.00 to 4.50% molybdenum, from 1.20 to 3.00 vanadium, from 0.50 to 5.00% cobalt, balance essentially iron and impurities, or containing, in weight percent, from 0.25 to 0.60% carbon, not more than 0.60% silicon, from 0.50 to 1.50% manganese, from 0.50 to 1.50% nickel, from 1.50 to 3.50% chromium, from 0.50 to 3.50% tungsten, from 1.50 to 3.00% molybdenum, from 1.20 to 3.00% vanadium, from 0.50 to 5.00% cobalt, balance essentially iron and impurities. This tool steel affords excellent high temperature mechanical strength, toughness, high temperature wear resistance, and heat check resistance.

'United States Patent Okuno et al.

TOOL STEEL FOR HOT WORKING Inventors: Toshio Okuno, Yasugi; Rinzo Sasaki, Nagoya, both of Japan Assignee: Hitachi Metals, Ltd., Tokyo, Japan Filed: Nov. 27, 1974 Appl. No.: 527,740

Foreign Application Priority Data Nov. 28, 1973 Japan 48-132737 US. Cl. 75/128 B; 75/128 V; 75/128 W Int. Cl. ..C22C 38/44; C22C 38/46;

C22C 38/52 Field of Search 75/128 B, 128 W, 128 V References Cited UNITED STATES PATENTS Primary ExaminerC. Lovell Assistant Examiner-Arthur J. Steiner Attorney, Agent, or Firm-Stewart and Kolasch, Ltd.

[57] ABSTRACT A tool steel for hot working containing, in weight percent, from 0.25 to 0.60% carbon, not more than 0.6% silicon, from 0.50 to 1.50% manganese, from 0.50 to 1.50% nickel, from 1.50 to 3.50% chromium, from 2.00 to 4.50% molybdenum, from 1.20 to 3.00 vanadium, from 0.50 to 5.00% cobalt, balance essentially iron and impurities, or containing, in weight percent, from 0.25 to 0.60% carbon, not more than 0.60% silicon, from 0.50 to 1.50% manganese, from 0.50 to 1.50% nickel, from 1.50 to 3.50% chromium, from 0.50 to 3.50% tungsten, from 1.50 to 3.00% molybdenum, from 1.20 to 3.00% vanadium, from 0.50 to 5.00% cobalt, balance essentially iron and impurities. This tool steel affords excellent high temperature mechanical strength, toughness, high temperature wear resistance, and heat check resistance.

4 Claims, No Drawings TOOL STEEL FOR HOT WORKING BACKGROUND OF THE INVENTION 1. Field of The Invention This invention relates to a tool steel for hot working, which presents excellent high temperature mechanical strength, toughness, high temperature wear resistance and heat check resistance.

2. Description of The Prior Art Hitherto, AISI H19 base steels of a high W-V-Co alloy system find a wide application as materials having the highest high temperature mechanical strength for meeting a demand arising from hot working which requires excellent mechanical strength and wear resistance at high temperature.

However, such steels suffer from disadvantages of often causing early stage or premature cracks due to the insufficient toughness in contrast to the excellent high temperature mechanical strength.

It is a principal object of the present invention to provide a tool steel for hot working-which present excellent toughness, high temperature wear resistance, heat check resistance and long service life superior to those of AISI H19 steel in addition to excellent high temperature mechanical strength similar to that of AISI H19 steel.

SUMMARY OF THE INVENTION According to the present invention, there are provided a tool steel for hot working containing, in weight 2 mium, from 0.50 to 3.50% tungsten, from 1.50 to 3.00% molybdenum, from 1.20 to 3.00% vanadium, from 0.50 to 5.00% cobalt, balance essentially iron and impurities. This tool steel affords excellent high temperature mechanical strength, toughness, high temperature wear resistance, and heat check resistance.

DETAILED DESCRIPTION OF THE INVENTION The tool steel according to the present invention has a composition being added suitable amounts of carbide forming elements such as W, Mo and V to a low Crlow SiMnNi-Co alloy system which is the fundamental components of this steel. In other words, the invention of this tool steel is: to improve resistance to the temper softening as well as high-temperature strength due to precipitation hardening by adding a small amount of chrominum, a suitable amount of tungsten and molybdenum and a great amount of vanadium; to present a suitable degree of oxidative characteristic due to the addition of small amounts of chrominum and silicon, suitable amounts of molybdenum and manganese to thereby facilitate the formation of oxide films on the surface of a die due to the temperature rise in service; and to provide the aforesaid oxide film which is intimate and adhesive, due to the addition of cobalt, nickel or tungsten, .in an attempt to improve high-temperature wear resistance, corrosion resistance, roughsurface condition and heat check resistance due to the resulting good lubricity and good heat insulating property; and to provide high resistance to the development or propagation of cracks by addition nickel to the composition of the steel.

percent, from 0.25 to 0.60% carbon, not more than 0.6% silicon, from 0.50 to 1.50% manganese, from 0.50 to 1.50% nickel, from 1.50 to 3.50% chromium, from 2.00 to 4.50% molybdenum, from 1.20 to 3.00% vanadium, from 0.50 to 5.00% cobalt, balance essentially Table 2 illustrates the heat treatment condition (target hardenss of HRC) and the high-temperature strengths, proving that the steels according to the present invention afford high-temperature strengths superior or equivalent to those of the conventional steels iron and impurities; and a tool steel containing, in (AISI H19).

Table 2 High-temperature Strength (700C) Present Invention Steel A Steel B Steel C Steel D Prior Art Steel E Heat Treatment Condition 0.2%

yield tensile strength strength (kg/mm") kg/mm quenched 1 150C hardened, 650C tempered 38.2 53.9

1140C 650C 36.9 52.1 I C 650C 37.4 52.7 1130C 650C 35.9 51.2

I140C 650C 37.8 53.0

weight percent, from 0.25 to 0.60% carbon, not more than 0.60% silicon, from 0.50 to 1.50% manganese. from 0.50 to 1.50% nickel, from 1.50 to 3.50% chro- Table 3 shows critical seizing loads (ratios) in the high'temperature seizing tests of steels according to the present invention.

protective and heat insulating properties of an oxide Table 3 film formed on the surface of the steel according to the 'ji jk z fi'f present invention.

06 r I Now, description will be given of the background of Present l' Steel A the composition of the steels according to the present Present Invention, Steel 8 l35 Present Invention, Steel C 145 lI'lVelltlOn. Prgsent Invention. Steel D 125 The amount of carbon can vary between 0.25% and Steel E 100 0.60%, preferably,0.30 and 0.50%. Carbon is necessary for maintaining desired hardenability, tempered Th h h f 10 hardness and high temperature hardness for steels aceat Freanjnem con moms remam t 6 Same or cording to the present invention. Carbon combines Samples as gwen m m In thpse F Samples Ofa with carbide forming elements such as tungsten, vanacolumn form were sub ected to an oxidation treatment dium and chrominum to thereby form carbides thus at 630 C beforehaqd after whlch .Samples rOta.nng contributing to formation of fine crystals, desired wear were pressed on end fases agamst a Steel 1.316.526 resistance property, temper softening resistance and (goumerpart) f r h i s m high temperature hardness of the steel. If the amount of t e 0a (crmcal 9 W wouid not carbon is excessive, then there results decrease in cause seizing, and then the critical loads were given as toughness and thus the amount of carbon Should be indexes in terms of the critical seizing load of a prior art not more than 060% on the other hand if the amount steel bemg A815 apparent from this tame the Steel of carbon is too small, then there results no intended accorqlpg the prefsgntjmvention presenis a high effect of the addition of carbon, and thus the amount of cal seizing oad. This is ue to the protective and lubri- Carbon Should be not less than 025% Catmg actlons of an oxlde Intimate and adheswe Silicon is present in amounts of not more than 0.60%.

which has been.formed on the sufface a sample. of Silicon has a tendency to impair the characteristic to the steel according to the present invention, presenting form heat insulating oxidized film formed due to the one of the prominent features of the present invention. temperature rise in a die in Service, and thus the Table 4 Indicates the fracture toughness (ASTM amount of silicon should be minimized in this sense.

5 type Sample) of the steels of the present mven' However, considering the factors involved in a steel making process, the amount of silicon should be pres- Table 4 ent in the above range.

The amount of manganese is between 0.50 and F cture t h k /mm m Dug g mm 1.50%. Manganese should be present for improving the l' l Sm] A characteristic to form a heat-insulating-oxidized film, Present invention, Steel B 262 Present invention, Steel (3 9 that IS, one of the prominent features of the present Present invenfim" Steel D 267 invention. lf manganese is excessive in amounts, then Prior art, Steel E 237 there results lowered A transformatlon point and higher tempered hardness as well as lowered machin- The heat treatment conditions used are the same as i the upper 1mm of the amolint of manganese those given in Table 2. According to the present inven- 40 IS Set to on the either hand If the amount of tion nickel is added so as to promote the resistance manganese P themtended f of manganese g h the development or propagation of Cracks can not be attained. Thus, the lower limlt of the amount presenting apparently higher fracture toughness than g 1 5821}. th f O 50 t l 5 0 does the prior art steels, as well as a tendency not to 1c S p n m 5 range 0 o cause cracks. This is also one of the prominent features Nlckel 1S necessary affording excelkent hlgh of the present invention ature strength plus high toughness (which contribute to Table shows the results of tests given to samples of the reslstance agfamst the dekelopmem k as steels of the present invention for their heat check pi 3 l fi i an i i p g oxldkzed resistances, in which samples of dimensions of 15 mm X i gl g z g z ggf iggi gg g fggf 25 mm were rapidly heated to 700C and then th f f H1 1 1 p quenched into water at 20C. Then such a cycle was pr.essmg Orma Ion o crac eve oping i euses repeated in 3 000 cycles with the aid of cobalt. If the amount of nickel 1s exces- As can be seen from this, the steels according to the there resplts lowered transformation present invention present excellent heat check resis- 1 1th the i gfi. fi r g q fi i t eamounto IllC e h h t o e urvalent to those of the nor art W 1C owers mac ma Hty tame lg er han r q p should be not more than 1.50%. On the other hand, if

eels. St the amount of nickel is too low, then there results no Table 5 desired effect of nickel added. Thus, the amount of Number Average depth Maximum depth nickel should be not less than 0.50%. of cracks of of cracksflnm) Chromlnum should be present within the range of prcseminvemion from 1 50 to 3.50%, preferably, from 2.00 to 3.50%. Steel A I 12 0.14 M 1 chrominum is necessary for improving the high temper- H4 (H4 057 ature strength and temper softening resistance as well Prior art steel E 120 0.18 0.65 as for affording a suitable characteristic of an oxidized film plus improved wear resistance due to the formation of carbides in combination with carbon. Chromi- The results shown above are attributable to the excelnum is further necessary for improving A, transformalent high temperature strength, and high resistance tion point effect and hardenability. In this sense, against the development or propagation of cracks plus chrominum is of supreme importance as an element for the steels of the present invention. If the amount of chrominum is too low, then there resultsinsufficient oxidation resistance, with theresulting rough surface, lowered hardenability, .lowered A, transformation point and lowered wear resistance. Thus, the lower limit of the amount of chrominum should be 1.50%. On the other hand, if the amount of chrominum is excessive, then there results excessively high oxidation resistance, so that the steel fails to form a protective oxide film which is one of the major features of the present invention. In addition, chrominum of an excessive amount promotes precipitation and cohesion of car bides, thus lowering the temper softening resistance and high temperature strength. Thus, the upper limits of chrominum is set to 3.50%.

Tungsten is present within the range of from 0.50% to 3.50%, preferably, from 1.00 to 3.50%.

Tungsten is necessary because of its unique effect to improve the wear resistance due to the formation of carbides which hardly forms solid solutions upon heating for hardening as well as to increase the high temperature yield strength due to the precipitation of fine carbides in tempering, in addition to an attempt to provide an intimate oxidized film on the surface of a die due to a temperature rise in service. The effect of tungsten on the characteristics of an oxide film depends on the amounts of Cr, Mo, Si, Mn, Ni, Co. Thus, optimum combination of those elements can present excellent characteristics as shown in Table 3. Tungsten of an excessive amount tends to form large size carbides, thus causing lowered toughness. Thus, the amount of tungsten is limited to not more than 3.50%. On the other hand, tungsten of a too small amount, there results the failure to obtain the intended effect of tungsten. Thus, tungsten should be present in amounts of not less than 0.50%.

Molybdenum is present in the range of from 2.00 to 4.50%. Molybdenum is necessary for forming carbides to increase wear resistance and for improving the hardenability due to the formation of solid solution thereof into a matrix. In addition, molybdenum precipitates fine carbides at the time of tempering to increase the temper softening resistance as well as high temperature strength, in addition to its feasibility to form a protec tive oxidized film. If the amount of molybdenum is excessive, then there results lowered toughness. Thus, the upper limit of molybdenum is set to 4.50%. On the other hand, if molybdenum is too low in amounts, then there results no intended effect due to the addition of molybdenum. For accommodating the combined addition of tungsten, the upper limit of molybdenum is set to 3.00%, and the lower limit thereof is set to 0.50%. In this respect, in the case of the addition of molybdenum accompanying no tungsten, the range of the molybdenum level is between 2.50 to 350% while in the case of combined addition of molybdenum with tungsten, the amount of molybdenum should range from 1.00 to 3.00%.

Vanadium is present within the range of from 1.20 to 3.00%, preferably, from 1.20 to 2.50%.

Vanadium is necessary-for forming a great amount of carbides which hardly form solid solutions to thereby improve the wear resistance and thermal stick resistance. In addition, vanadium fonns solid solutions in a matrix at the time of heating for hardening, thus precipitating fine carbides which are hardly cohesive in tempering. This increases the softening resistance at elevated temperatures thus affording excellent high-temperature yield strength, which is one of the outstanding features of the present invention. For this reason, vanadium should be present in a great amount for the steels of the present invention. In addition, the addition of vanadium presents a fine grain size ofcrystals to improve toughness as well as to raise the A, transformation temperature, thus improving high-temperature yield strength at elevated temperatures as well as heat check resistance. If the amount of vanadium is excessive, then there results large size carbides, thus lowering toughness. Thus, the upper limit of vanadium is set to 3.00%. If the amount of vanadium is too low, then there results no intended effect due to the addition of vanadium. Thus, the lower limit of vanadium is set to 1.20%.

Cobalt is present within the range of from 0.50 to 5.00%, preferably, from 0.70 to 4.00%. Cobalt is necessary for affording excellent wear resistance at elevated temperatures. This is because of the formation of a protective oxide film which is intimate and adhesive, at the time of the temperature rise in a die in service. This prevents its metallic contact with work pieces, preventing the temperature rise in a die as well as presenting high wear resistance. In addition, cobalt presents a heat insulating property due to the formation of an oxide film, improvement in heat check resistance due to the protective action of the film, and suppression of formation of crack developing nucleuses. The aforesaid effect of cobalt differs by the added amount of nickel, tungsten, molybdenum and the like. Thus, in the case of the present invention, cobalt should not be added in a great amount. If the amount of cobalt is excessive, then there results lowered toughness. Thus, cobalt should be present in amounts of no more than 5.00%. On the other hand, if the amount of cobalt is too low, then there may not attain the intended effect due to the addition of cobalt. Thus, cobalt should be present in amounts of no less than 0.50%.

As is apparent from the foregoing description, the steels for hot working according to the present invention present high resistance against the development or propagation of cracks, excellent high wear resistance and seizing resistance due to the formation of a dense and adhesive oxide film by temperature rise at hot working. In addition, the steels of the present invention suppress the formation of the initial cracks or crack developing nucleuses as well as insure a long service life with consistent high performances.

What is claimed is:

l. A tool steel for hot working consisting essentially of, in weight percent, from 0.25 to 0.60% carbon, not more than 0.6% silicon, from 0.50 to 1.50% manganese, from 0.50 to 1.50% nickel, from 1.50 to 3.50% chromium, from 2.00 to 4.50% molybdenum, from 1.20 to 3.00% vanadium, from 0.50 to 5.00% cobalt, balance essentially iron and impurities.

2. A tool steel for hot working consisting essentially of, in weight percent, from 0.03 to 0.50% carbon, no more than 0.60% silicon, from 0.50 to 1.00% manganese from 0.70 to 1.40% nickel, from 2.00 to 3.50% chromium, from 2.5 to 3.50% molybdenum, from 1.20 to 2.50% vanadium, from 0.70 to 4.00% cobalt, balance essentially iron and impurities.

3. A tool steel for hot working consisting essentially of, in weight percent, from 0.25 to 0.60% carbon, not more than 0.60% silicon, from 0.50 to 1.50% manganese, from 0.50 to 1.50% nickel, from 1.50 to 3.50% chromium, from 0.50 to 3.50% tungsten, from 1.50 to 3.00% molybdenum, from 1.20 to 3.00% vanadium,

from 0.50 to 5.00% cobalt, balance essentially iron and impurities. 4. A tool steel for hot working consisting essentially of, in weight percent, from 0.30 to 0.50% carbon, no more than 0.60% silicon, from 0.50 to 1.00% manga 

1. A TOOL STEEL FOR HOT WORKING CONSISTING ESSENTIALLY OF, IN WEIGHT PERCENT, FROM 0.25 TO 0.60% CARBON, NOT MORE THAN 0.6% SILICON, FROM 0.50 TO 1.50% MAGANESE, FROM 0.50 TO 1.50% NICKEL, FROM 1.50 TO 3.50% CHROMIUM, FROM 2.00 TO 4.50% MOLYBDENUM, FROM 1.20 TO 3.00% VANADIUM, FROM 0.50 TO 5.00% COBALT, BALANCE ESSENTIALLY IRON AND IMPURITIES.
 2. A tool steel for hot working consisting essentially of, in weight percent, from 0.03 to 0.50% carbon, no more than 0.60% silicon, from 0.50 to 1.00% manganese from 0.70 to 1.40% nickel, from 2.00 to 3.50% chromium, from 2.5 to 3.50% molybdenum, from 1.20 to 2.50% vanadium, from 0.70 to 4.00% cobalt, balance essentially iron and impurities.
 3. A tool steel for hot working consisting essentially of, in weight percent, from 0.25 to 0.60% carbon, not more than 0.60% silicon, from 0.50 to 1.50% manganese, from 0.50 to 1.50% nickel, from 1.50 to 3.50% chromium, from 0.50 to 3.50% tungsten, from 1.50 to 3.00% molybdenum, from 1.20 to 3.00% vanadium, from 0.50 to 5.00% cobalt, balance essentially iron and impurities.
 4. A tool steel for hot working consisting essentially of, in weight percent, from 0.30 to 0.50% carbon, no more than 0.60% silicon, from 0.50 to 1.00% manganese, from 0.70 to 1.40% nickel, from 2.00 to 3.50% chromium, from 1.00 to 3.50% tungsten, from 1.00 to 3.00% molybdenum, from 1.20 to 2.50% vanadium, from 0.70 to 4.00% cobalt, balance essentially iron and impurities. 